This study assessed the hypothesis that average motor unit firing rates change in parallel with the contractile properties of the vastus lateralis following eight weeks of isometric resistance training. The firing rates of 400+ motor units in the vastus lateralis were obtained during voluntary isometric contractions of 50% MVC, before and again after training in males (N = 10; only one leg was trained) and untrained controls (N = 10).

Training resulted in an MVC increase of 36%, and a maximal instantaneous rate of contraction increase of 20% in the trained leg. Neither the maximal integrated EMG nor the rate of increase of integrated MEG was different after training. No significant change occurred in the untrained leg or the control group. The hypothesis was not supported.

Resistance-training produced a large increase in maximal voluntary contractile force and a small increase in contractile speed of the knee extensors. There were no accompanying changes in neural activation at a motor unit firing rate of 50% MVC. These results suggest that a short period of isometric training increased maximal force production but it did not result from enhanced activation of the whole muscle. Baseline motor unit firing rates were already high enough to maintain precise control of sustained submaximal forces despite increases in contractile speed resulting from the resistance training.

Implication. Extended periods of high-load resistance training that produce marked changes in voluntary strength may contribute little to the performance of activities that require submaximal strength levels. Such activities are sports that require fast movements (e.g., rowing, kayaking, swimming) where large-force generation does not have time to occur, or sports that require many submaximal repetitions (e.g., distance running, cycling, games requiring intermittent bursts of activity such as soccer). The energy misdirected into non-beneficial strenuous resistance training could better be used for sport-specific training.